Optical communication systems have recently become widespread, and optical amplifiers are being used in various communication devices. For example, optical amplifiers are used in optical transmission devices, such as an optical transmitter, an optical receiver, an optical repeater and an optical add-drop multiplexer.
When used in optical transmission devices as mentioned above, optical amplifiers sometimes operate in an automatic gain control (AGC) mode in which an optical signal is amplified while the gain is maintained to be fixed. In the AGC mode, for example, the power of light input to an optical amplification medium (hereinafter referred to as “input light power”) and the power of light output from the optical amplification medium (hereinafter referred to as “output light power”) are monitored, and excitation light is controlled so that the ratio of the input light power to the output light power (that is, gain) is maintained to be fixed. Accordingly, when the input light power decreases, the control system of the optical amplifier reduces the power of excitation light so that the gain is maintained to be fixed. Similarly, when the input light power increases, the control system of the optical amplifier increases the power of excitation light so that the gain is maintained to be fixed.
An optical amplifier performs control as mentioned above and therefore is designed to include many optical components. When any one optical component breaks down, it becomes impossible for the optical amplifier to amplify an optical signal so as to obtain desired gain. For this reason, methods for detecting a failure in an optical amplifier have been proposed.
For example, when a laser light source for generating excitation light of an optical amplifier deteriorates, a control signal for increasing the power of excitation light is provided to the laser light source in order to maintain fixed gain. Accordingly, in this configuration, breakdown of a laser light source (or deterioration of the laser light source) is detected if the control signal mentioned above is monitored. In addition to this, for example, a method in which a failure in an optical amplifier is detected based on a change in output light power or a change in gain is known.
Japanese National Publication of International Patent Application No. 11-507189 and Japanese Laid-open Patent Publication No. 2003-174420 disclose examples of the related art.
If the input light power of an optical amplifier varies, the control system controls excitation light power in accordance with the variation of the input light power in order to maintain the gain to be fixed. In some cases, however, it is not possible to determine whether such a change in excitation light is caused by a variation in input light power or caused by a failure in an optical amplifier, only by monitoring excitation light or a control signal for generating excitation light. That is, with the related art technologies, whether a failure has occurred in an optical amplifier is not able to be correctly determined in some cases.